The present invention relates to a method for monitoring the consistency of the grease manufactured by a continuous grease making process. Continuous grease making processes were developed in the early 1960's. Their refinement has been vigorously pursued due to the potential advantages the continuous process offers over the widely used batch process for grease manufacture. Some understanding of the batch process is required in order to fully appreciate the advantages of the continuous grease making process. The batch method is currently the most widely used method for grease making. Batch processing is carried out with equipment having widely varying capacities. That capacity can range from 50 pounds of grease to about 20,000 pounds of grease. The equipment involved can be expansive taking up two floors of a building and employing a large operating crew. The major components of a typical batch process are a contactor, a kettle, and various kinds of finishing equipment. The contactor is a pressure vessel wherein the thickener that provides the structure to hold the lubricating oil contained in the grease is formed. The thickener most commonly used for modern grease production is some sort of metallic soap made from a fatty acid, a metal base, water, and in some cases a small amount of lubricating oil. The soap is usually referred to by the name of the metal base used to make the soap. Commonly used metals used include aluminum, lithium and barium. These components undergo a saponification process whereby they are mixed in the presence of heat and pressure to form the soap thickener. Greases referred to as complex greases were developed when it was discovered that different kinds of fats could be combined to make greases. As the art of grease making has evolved over the years, the term complex grease has been used for many different purposes but today has generally been accepted to refer to a high temperature application grease.
From the contactor the soap mixture is transferred to a kettle for dehydration and the addition of additional oil stock. The soap structure leaving the contactor is quite wet and as in any grease manufacturing process requires removal of excess water. The soap mixture will remain in the kettle for many hours where the mixture is heated and water vapor is drawn therefrom. The kettles typically contain internal agitators or scrapers that serve to break up the soap structure to improve the consistency and workability of the grease. The scrapers also remove masses of soap from the sides of the kettle. The kettles are heated by steam, electricity or via the circulation of some type of hot oil. The type of heat source utilized will vary with the maximum temperature required in order to form the grease.
From the kettle the grease is pumped to various kinds of finishing equipment. This finishing equipment is required to ensure that the soap structure and any grease additives are evenly distributed throughout the grease. This equipment includes milling machines which break up the fibrous structure of the soap and homogenizers which improve the dispersion of the soap in the grease. The finishing step could also include filtration to remove impurities or deaeration to remove entrained air. Air is introduced into the grease while it is beaten in the kettle. Excess air can cause problems with the appearance of the grease and can prevent the required weight of grease from being introduced into the intended packages. The grease is then cooled and packaged.
A number of variations on the batch process are possible to include changing the size of the kettle used and carrying out the entire process inside one vessel. However, a common characteristic of all batch processes is that the grease is manufactured in discrete units in a discontinuous fashion.
By contrast continuous grease production units take up a fraction of the space required by batch processing equipment and can be operated by a small complement of operators. The continuous production process has proven capable of achieving a higher output of a consistently high quality product. Moreover, this process results in less wasted product created during the changeover from one grease to another.
An early continuous grease production method was described in U.S. Pat. No. 3,475,335 to Greene et al. That process comprised continuously introducing a saponifiable material and a metal base into a tubular reaction zone at high temperature and pressure under turbulent conditions to obtain substantially complete reaction. Next a lubricating oil is introduced into the product stream that is continuously withdrawn from the reaction zone. The product stream continues to a dehydration zone wherein the grease mixture is maintained at an elevated temperature but below the melting point of the soap and under a pressure substantially lower than the pressure in the reaction zone. The grease is maintained in the dehydration zone for a period sufficient to substantially dehydrate the mixture. The product stream out of the dehydration zone is partially recycled back into that zone through a shear valve which serves to condition the soap fibers contained in the grease. The product stream is then additized and passed to coolers and possibly through additional conditioning steps.
U.S. Pat. No. 4,297,227 to Witte et al. describes an improvement to a continuous grease making process which permits the use of water soluble additives in grease compositions. The improvement permits the incorporation of such additives in an evenly dispersed fashion thereby eliminating the need for a separate step to form an additive slurry. In this improved process a saponifiable material and a metal base are continuously introduced at elevated temperatures and superatmospheric pressure into a saponification zone where they are saponified under turbulent conditions. The saponified product is then mixed with an aqueous solution of water soluble additive materials at superatmospheric pressure sufficient for maintaining all water in the liquid phase. The product is then dehydrated by flash vaporizing substantially all the water therefrom. The resulting grease has a water soluble additive evenly dispersed therethrough as particles not exceeding about 10 microns in size.
A process for the continuous production of high dropping point lithium complex soap greases is disclosed in U.S. Pat. No. 4,444,669 to Wittse, Jr. et al. The thickener used in that invention is a mixture of lithium soaps of hydroxy monocarboxylic fatty acids and dicarboxylic fatty acids. The patent notes that process conditions must be very closely controlled in order to produce the desired high dropping point greases.
As stated herein above, a critical requirement for the successful utilization of continuous grease processes has been the need to monitor product quality during production. The high production rates possible with these units creates the possibility that a large amount of off-specification product can be produced between the time that a problem is detected and the time that production is shut down. Monitoring is particularly important during start up of the operation. During the time required for adjustment and stabilization of grease flow a great deal of unusable product may be produced. Such a large volume of unusable product can make the continuous process an extremely uneconomic venture. Therefore it is critical to be able to monitor the quality of the effluent stream of a continuous grease production unit to determine the instant when the transition from unacceptable to acceptable product occurs.
The parameter most often monitored is the grease consistency. This term has been used to refer to the texture or elasticity of a grease, however, it is now accepted to refer to the degree to which a grease will deform upon the application of a force. Consistency is measured using the cone penetration test specified in ASTM D217, "Cone Penetration of Lubricating Grease." The test is conducted using an ASTM penetrometer and is widely known by skilled operators. Accordingly the details of the test procedure will not be repeated here. The test is simple easy to conduct and provides reproducible results. Test results are reported in the number of tenths of millimeters to which a standard cone sinks into a grease under prescribed conditions. As the penetration number increases so does the softness of the grease. In the context of a continuous grease process, however, the test requires a great deal of time to conduct. Quite a large volume of unusable grease can be produced during the time required to take a sample and perform the penetrometer test. Accordingly if grease will be made by a continuous process, it must be monitored by a continuous process.
An apparatus for the continuous monitoring the consistency of a lubricating grease stream is disclosed in U.S. Pat. No. 4,043,183 to Higgs et al. That consistometer included a resilient member located in the stream of material and a reference member located outside of the stream. Both members rotate synchronously and are spatially related to sensors that detect the passage of the members. The reference member and resilient member generate reference and resilient pulses respectively which signals are fed to a network that provides an output corresponding to the consistency of the material.
While this consistometer performed adequately for simple greases, it has been found that certain types of polymeric additives used in complex greases cause the consistometer to give inaccurate readings. Those additives caused the grease to adhere very strongly to the resilient rotating member resulting in inaccurate outputs. Without accurate, timely information on the consistency of the grease as it is being produced, quite a bit of guess work is required to use the continuous process. As a result, an economically unacceptable amount of off-specification grease would be produced unless the manual penetrometer test described herein above is performed. Therefore, there exists a need for a reliable means to monitor the consistency of the effluent stream of a grease making process.